(1) Field of the Invention
This invention concerns improvements in and relating to traffic data acquisition which includes weight reporting and data which may be processed for law enforcement and for road engineering.
(2) Prior Art
Sophisticated equipment has been developed for traffic data processing and law enforcement. This equipment is based on coaxial cables exhibiting piezo-electric and/or tribo-electric effects, loop detectors and axle weight pads. Weight measurement of vehicles at speed has in particular been difficult and the state of the art weight measurement pad developed from the technology described in South African patents 68/4975 and 69/1840 is cumbersome and costly. The weight pad has a further problem in that it does not report footprint area of the vehicle wheel so that the pressure on the road (which is the criterion of interest to road design engineers) cannot be directly reported nor reliably computed. One of the present inventors has been aware from an early stage that the comparatively cost effective coaxial cable developed from technology described in South African patent No. 66/0934 does exhibit a weight sensitive response. However, these PVC based coaxial cables could never be used for weight measurement because of problems from the cable itself and from limited signal processing capabilities or means. The cable of the time exhibited problems that still exist today (in any dynamic weight measuring system) but the extent and influence of these errors were enhanced by the then poor production processes, quality controls and technologies. These problems could be described as scatter, correlation, temperature dependence, speed dependence, cross-wise dependence and general sensitivity (poor signal to noise ratio). Scatter described the phenomenon whereby the same axle weight driven at the same speed in the same cross-wise position and at the same temperature in successive readings gives different peak or peak-to-peak outputs from the standard PVC based coaxial cables. It has not been feasible to identify the origin of this scatter. Traffic measurement equipment is exposed to severe temperature extremes, for example, from sub-zero temperatures to 80.degree. C. and the electrical signal received from such cables is heavily temperature dependant. Even software and/or hardware based temperature compensation technique and calibration of each particular cable has not been able to provide a satisfactory solution of this problem. The standard PVC coaxial cable also exhibited speed dependance, the peak value of the pulse rising with rising speed but again with a significant statistical component or scatter so that this problem too could not be satisfactorily solved by means of a calibration based approach. Finally the state of the art PVC coaxial cable frequency exhibited cross-wise dependence, that is, the precise position along the length of such a cable stretched across the road at which the vehicle wheel passed over it should be controlled, which in practice was impossible.
This great variation in the general sensitivity of the cable made signal/noise ratio adjusting very difficult. The signals produced from light vehicles at low speeds normally fell in the noise regions making detection of passing axles very difficult at low speeds. Signal processing means involved standard transistor technology coupled with TTL integrated circuits of the time. Although certain acceptable levels of correlation were found between the voltage peak parameter and dynamic axle weight, this varied from detector to detector and site to site. This brought into question production repeatability and calibration requirements. The calibration value was also found to be dependent on temperature, installation method and life span since the cable's uniformity changed due to its mechanical stress characteristics along its length. This added to the inherent cross-wise dependability of the system. More tests involving signal integration and differentiation, each involving different installation methods, were unsuccessful in bettering the axle weight to voltage signal correlation values.
U.S. Pat. No. 4,712,423 to Siffert et al. discloses the use of piezo electric cables to determine the weight and speed of vehicles which cross a cable. Siffert et al. shows an analogue processing circuit and carries out a linear integration of the signal from the cable which gives a result which is influenced by the polarity reverses of the signal. This sums the positive and negative areas under the curve representing the signal and gives a net total.
All signal processing is required in real time and this limited the use of early micro-computer technologies, apart from their general power requirements and availability, the only high frequency (high speed processing) devices. Early micro-computer trials were unsuccessful due to the low reliability under exposed conditions and high development costs.